what is the average number of miles before an engine fails

Hello, there is no real answer for this! There are so many variables here. What has failed in your engine? All you can do is change your engine oil and oil filter regularly with top quality products to ensure proper filtration and lubrication. You also want to maintain your engine air filter and ensure good fuel is being used. Depending on why your engine "failed" - it may be easier to explain possible causes. But at some point, everything does wear out.